The invention relates generally to replacing a bladeless closure piece in a turbine wheel to improve the efficiency of operation of the turbine wheel and more specifically to providing a fully bladed closure design for tangential entry round skirt dovetails.
Steam turbine blades, or buckets, are often designed for installation on a turbine wheel in a tangential direction. The buckets are typically attached to the turbine wheel using external circumferential dovetails, with a male dovetail on the wheel periphery (margin) and a complimentary female dovetail in the base or root of the bucket. In order to load these buckets onto the wheel, a notch which locally removes the male dovetail portions is cut on the periphery of the wheel, leaving a generally rectangular core portion. Each bucket is then initially located over the core material in the notch and then displaced tangentially onto and around the wheel. The last bucket to be assembled to the wheel is called the closure block. Once all the buckets have been loaded, a closure block is utilized that is formed with laterally spaced tangs extending radially inwardly and that are adapted to straddle the core material in the notch. The closure block is secured by a retaining pin passing through the tangs and core. In this way, the buckets on the wheel are locked in place and thus prevent the buckets from moving circumferentially along the dovetail.
Front or first stage turbine buckets are subjected to high temperatures over 900 degrees F. Limitations of material stress capability mean that only a lightweight block, which has no airfoil, can be used as the closure block, causing reduced performance. Buckets for other stages may also be subjected to high temperatures and great stresses. Because the closure block has no airfoil, there is an opening in the steam path with detrimental effects on performance of the wheel. The reason behind the inability to support an airfoil on the closure bucket is the fact that the retaining pin passes through the core material in the highly stressed dovetail region of the wheel. There is thus a need for a closure block with a mounting or retaining arrangement that provides sufficient strength to permit the incorporation of an integral airfoil that closes the opening, thus producing greater performance.
Referring to FIG. 1, a typical turbine rotor or wheel 10 (partially shown) includes a male dovetail configuration 12 formed about the periphery of the wheel, with upper and lower axial projections 14, 16 (projecting outwardly from both sides of the wheel) as conventionally provided. A notch (or insertion gap) (not shown) with a width adequate to permit the female dovetail portion of buckets to slide over is provided. Further, an axial oriented hole 25 is provided through the notch. Buckets 18 having an airfoil 20, a platform 22 and a root or base portion 24 are shown loaded onto the wheel 10. It will be understood that the closure block is the last of a circumferential row of buckets to be loaded on the wheel. The closure block 26 is shown inserted over the notch, formed by removing the projections 14, 16 on opposite sides of the dovetail. A pair of tangs (one shown at 30) straddles the remaining core material of the dovetail. A retaining pin 32 is press fit into aligned openings in the core and the tangs 30. Because the stresses at the location of pin 32 are high, the closure block 26 cannot support an airfoil, and thus an undesirable space 34 is left unfilled.
The closure bucket typically does not have a dovetail to provide support because a dovetail would be useless in the notched space that the closure bucket occupies. Therefore, the closure bucket must be secured by other means. Various arrangements have been attempted to provide a bladed closure bucket.
One approach by Reluzco et al. (U.S. Pat. No. 6,499,959) was to fix the closure bucket to adjacent buckets, i.e., the two buckets that straddle the gap and the closure bucket, in order to secure the closure bucket to the wheel. Typically, the closure bucket is attached to the adjacent buckets by pins extending in an axial direction engaging through the root or base portions of the adjacent buckets and the closure bucket. Here, the centrifugal load of the closure bucket is carried by the adjacent buckets through the pins. The applied loads on the closure and adjacent buckets are thus not uniform. High localized stresses are encountered at the location of the securing between the closure bucket and the two adjacent wheels, i.e., along the slots receiving the pins and the pins themselves. Consequently, creep and permanent deformation of the closure bucket and/or the adjacent buckets may occur after a period of operation at high temperatures and high centrifugal loads. For example, such high temperatures and loadings may occur in the reheat section of an intermediate stage turbine. As a result, the closure bucket may tend to elongate at its base or root in response to these high temperatures and stresses over time, with the result that the slot or hole for receiving the pins may elongate in a radial outward direction. Consequently, there is a need for an increase in the load-carrying capacity of at least the closure bucket in a steam turbine.
To avoid creep failure, the closure bucket and preferably the two adjacent buckets are formed of a material having a higher strength, e.g., a higher creep rupture strength than the creep rupture strength of material forming the remaining buckets. For example, the remaining buckets may be typically formed of a stainless steel. The material of the closure and adjoining buckets, however, may comprise a nickel-based alloy and more particularly and preferably an Inconel-based alloy. Additionally, the pins 32 are preferably formed of a material having a higher creep rupture strength than the creep rupture strength of the remaining buckets. Thus in Reluzco, the pins are preferably formed of a similar material as the closure and adjacent buckets, although it will be appreciated that the pins may be formed of a different material having a higher creep rupture strength than the creep rupture strength of the stainless steel buckets.
In Munshi et al. (U.S. Pat. No. 6,755,618), another method for supporting a fully bladed closure block was provided. Here , as shown in FIG. 2, the notch for loading buckets onto the turbine wheel 38 was cut deeper in the radial direction than the typical notch for a closure. Again, the periphery of the wheel is formed with a male dovetail 40 including projections 42, 44 that cooperate with complimentary female dovetails (not shown) formed in the buckets. The closure bucket 36 is inserted onto notch 46, after all of the other buckets in the row are installed. The notch 46 at the bucket loading location is deeper in a radial direction than typically formed notches (like the notch described for FIG. 1), and the closure bucket 36 is formed with a root portion 47 that includes extended radial tangs 48, 50, each provided with a pair of radially aligned holes 52, 54 (one pair shown on tang 50). Holes 52, 54 of one tang are also axially aligned with the holes in the other tang. Because of the extended radial depth of the notch 46 and tangs 48, 50, radially aligned retaining pins 56, 58 used to secure the closure bucket 36 pass through the core of the wheel 38 entirely radially inside the dovetail 40 formed on the periphery of the wheel, providing extra support for the closure bucket to have an integral airfoil 60. However, the deeper cut requires specialized machining.
Accordingly, there is a need to provide a fully bladed closure design for a turbine wheel with round skirt dovetails that can tolerate centrifugal loads and high temperatures without resorting to special strength material or making deep cuts on the notch in the peripheral margin of the wheel.